Summary of Work: Telomeres are structures on the ends of linear chromosomes that are required for genome stability. They serve at least two essential functions: to counterbalance the loss of DNA from a chromosome end due to the inability of DNA polymerase to replicate linear DNA completely, and to distinguish a natural chromosome end from a broken chromosome end. Telomeres are complex, having a tandem array at the end that appears to be important for both of these functions, a more complex array of repeats of unknown function proximally, and proteins that bind to these DNA repeats. The subtelomeric repeats are thought to be associated with the formation of heterochromatin and the inactivation of genes placed in their vicinity, and may be instrumental in forming associations with other telomeres, the nuclear matrix and the lamina. Reporter genes placed in subtelomeric repeats variegate, that is they are expressed in some cells of the appropriate tissue, but not others. The subtelomeric repeats from chromosome 2L have been ligated to a reporter gene and reintroduced into the genome to ask whether, and under what circumstances, this repeated sequence reduces gene activity. Repression occurs in a orientation and array-length dependent manner. Thus, this repeat array may be important for telomeric heterochromatin formation, but it does not cause variegation. Other components of the telomere are now being tested for the ability to modulate the effect of this subtelomeric repeat on gene expression. A telomeric minisatellite has also been obtained from the malaria vector, Anopheles gambiae, and is being tested for its effects on gene expression in this assay. In a separate approach to understanding telomere structure and telomeric silencing, trans-acting modifiers of variegating telomeric inserts will be isolated and characterized. Similar modifiers of centromeric position effect variegation have been used to identify heterochromatic proteins, but none of these modifiers have an effect on telomeric position effect. This suggests that an entirely new set of telomeric proteins may be identified by the proposed search.